Laser shock hardening method and apparatus

ABSTRACT

There is provided an improved laser shock hardening method and apparatus which can eliminate spattering of a liquid and waving of the liquid surface upon laser irradiation, and can stably irradiate a workpiece with a laser beam. Thus, the present invention provides in a laser shock hardening method for carrying out surface processing of a workpiece in contact with a liquid by irradiating through the liquid the surface of the workpiece with a pulsed laser beam intermittently emitted from a laser irradiation device, the improvement comprising: providing a solid transparent to the wavelength of the laser, serving as an entrance window to the surface of the liquid; allowing the liquid to be present in the light path of the laser beam between the solid and the surface of the workpiece; and allowing the laser beam to enter through the solid and irradiating through the liquid the surface of the workpiece with the laser beam, thereby shock-hardening the surface of the workpiece.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a Divisional of U.S. application Ser. No.11/431,920, filed May 11, 2006, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laser shock hardening method andapparatus for irradiating the surface of a solid material, such as ametal or a ceramic, with a pulsed laser beam through a liquid to adjustsurface or internal characteristics of the material, such as structure,hardness and residual stress.

2. Background Art

Defects in a structure, such as corrosion and cracks, in most casesoriginate from the surface, and the life of the structure depends on itssurface characteristics. Attempts have therefore been made to improvethe mechanical or chemical properties of the surface of a material, suchas fatigue strength, corrosion resistance and wear resistance, therebyprolonging the life of the structure.

Shot peening is a typical surface processing technique. This techniqueenables a rise in the hardness of a surface layer of a workpiece andintroduction of compressive residual stress into the surface layer, andtherefore is widely used in the industrial fields of automobiles,aircrafts, etc. (see, for example, “Metal Fatigue and Shot Peening”,edited by Society of Shot Peening Technology of Japan).

Laser irradiation, on the other hand, enables precise and high-speedcontrol of energy density and irradiation point and can carry outhigh-speed processing, rapid heating/quenching processing, etc. whichare difficult with other methods. Accordingly, various laser irradiationtechniques have been developed which find wider application toprocessing of materials.

One such technique is laser shock hardening which involves irradiationof the surface of a material with a pulsed laser beam through a liquid.As with shot peening, this technique enables a rise in the hardness of asurface layer of a workpiece and introduction of compressive residualstress into the surface layer.

Laser shock hardening has a higher effect than shot peening and, inaddition, has various excellent advantages that shot peening does nothave, such as capability of contactless operation, no involvement ofreaction force and capability of precise control of laser irradiationconditions and laser irradiation sites. Development and practicalapplication of this processing method are now under way (Japanese PatentLaid-Open Publications Nos. 7-246483, 8-112681, 8-326502, and2003-504212).

Laser shock hardening, which involves irradiating the surface of a solidmaterial, such as a metal or a ceramic, with a pulsed laser beam througha liquid to adjust surface of internal characteristics of the material,such as structure, hardness and residual stress, will now be describedwith reference to FIGS. 1 through 3.

FIG. 1 illustrates a method in which a workpiece 41 disposed in a liquid22 is irradiated with a pulsed laser beam 51 to adjust the materialcharacteristics, such as structure, hardness and residual stress, of theworkpiece 41.

When the peak power density of the laser beam 51 exceeds the plasmageneration threshold of the workpiece 41 (approximately 0.1 to 10 TW/m²in the case of a metal), the topmost surface layer (1 μm or lower) ofthe workpiece 41 evaporates instantly to generate a plasma 52. Becauseof inertia strongly acting instantaneously in the liquid 22, the plasma52 can little expand and the energy of laser beam 51 concentrates in anarrow area. Accordingly, the pressure of the plasma can even reach10-100 times the pressure in the air or in vacuum.

When water is used as the liquid 22, the pressure P (GPa) of the plasmagenerated is approximately equal to (0.2×I)^(0.5), wherein I (TW/m²)represents the peak power density of the laser beam 51 applied to theworkpiece 41. In case the liquid 22 is a liquid other than water, suchas an alcohol, ammonia water or a boric acid solution, the plasmapressure can be determined by the equation: P=(0.2×I×k)^(0.5),k=(acoustic impedance of liquid)/(acoustic impedance of water)

The “acoustic impedance of liquid” is equal to (density ofliquid)×(sonic velocity in liquid). With the above-described liquidother than water, therefore, the plasma pressure in the liquid does notdiffer significantly from that in water. Thus, in either case, when thesize and the pulse energy of the laser beam 51 are so controlled as tomake the peak power density of the laser beam 51 1-100 TW/m² at thesurface of the workpiece 41, the pressure of the plasma 52 will beapproximately 450 MPa-4.5 GPa.

The high-pressure plasma 52 thus generated instantaneously compressesthe surface of the workpiece 41 and the surface displacement caused bythe compression generates a shock wave 53 that propagates in the depthdirection of the workpiece. The shock wave 53, when its pressure exceedsthe yield stress of the workpiece, will cause a local plasticdeformation. This makes it possible to adjust the materialcharacteristics, such as structure, hardness and residual yield.

FIGS. 2 and 3 illustrate an example of adjustment of materialcharacteristics by laser shock hardening, FIG. 2 showing a change in thehardness of a stainless steel (SUS 304) and FIG. 3 showing a change inthe residual stress of the stainless steel. A laser beam of a pulseenergy of 200 mJ and a pulse width of 8 ns was collected such that theirradiation spot takes the shape of a circle having a diameter of 0.8 mmand was applied at 36 pulses per 1 mm², so that the peak powder densitybecame 50 TW/m². Reference numerals 71, 72 denote the hardness valuesbefore and after processing. The comparative data shows a rise in thehardness in the region nearly to the depth of 1 mm by the laser shockhardening processing. Reference numerals 73, 74 denote the residualstress values before and after processing. The comparative data shows animprovement from tensile to compressive in the residual stress in theregion nearly to the depth of 1 mm by the laser shock hardeningprocessing.

Such a rise in the hardness of the surface of a material and theformation of a compressive residual stress are effective in enhancingfatigue strength and preventing stress corrosion cracking. Therefore,laser shock hardening has been progressively employed in the aircraftindustry, the automobile industry, the atomic industry, etc.

Since laser shock hardening involves direct irradiation of the surfaceof the workpiece 41 with the pulsed laser beam 51, there is a case wherean element, constituting the liquid 22 decomposed by the plasma 52,reacts with the surface of the workpiece 41.

For example, in the case of laser shock-hardening a stainless steel in awater atmosphere, hydrogen and oxygen are generated by the decompositionof water, and the oxygen reacts with the surface of the stainless steel,whereby a strong black oxide film having a thickness of about 1 μm,composed mainly of Fe₃O₄, is formed on the surface after the processing.

In case such a black film is undesirable for its appearance, a coatingfilm having a thickness of the order of several tens of μm may be formedon the surface of the workpiece 41, for example with a paint or a metaltape, prior to laser shock hardening. After removing the coating film,the surface state of the workpiece 41 will be almost the same as thatbefore the processing.

In laser shock hardening, the surface of a material is irradiated with apulsed laser beam through a liquid, such as water. Upon irradiation witha laser beam, there occurs the phenomenon that a high-pressure plasma,generated on the surface of the material, spatters the liquid anddisturbs the liquid surface. When irradiation with the next laser beamis carried out shortly thereafter, the position or the shape of theirradiation spot can change due to refraction. The next laser beamirradiation should therefore be awaited until the disturbance of theliquid surface settles down, which precludes speeding up of theprocessing.

Further, laser shock hardening is generally carried out by applying apulsed laser beam, shaped into a circular or square shape of a size ofabout 1 to several mm, to the surface of a material. Laser shockhardening thus has the drawback that only a small area can be processedwith one pulse, that is, the processing speed is low. Studies havetherefore been made on methods for speeding up of processing, forexample, the use of a laser oscillator with a high repetition or the useof a laser oscillator with a large pulse energy. Such speeding-upmethods, however, entail such problems as the necessity of using alarger-sized laser oscillator or a larger-sized driving device formoving a workpiece or an irradiation head. Speeding up of laser shockhardening processing has thus been difficult.

SUMMARY OF THE INVENTION

The present invention has been made in order to solve the above problemsin the prior art. It is therefore an object of the present invention toprovide a laser shock hardening method and apparatus which enables arise in the hardness of the surface of a workpiece and an improvement inresidual stress in the surface, can eliminate spattering of a liquid andwaving of the liquid surface upon laser irradiation, and can stablyirradiate the workpiece with a laser beam.

It is also an object of the present invention to provide a laser shockhardening method and apparatus which makes it possible to process aworkpiece at a sufficiently high speed without imposing an excessiveburden on a driving device.

In order to achieve the above objects, the present invention provides ina laser shock hardening method for carrying out surface processing of aworkpiece in contact with a liquid by irradiating through the liquid thesurface of the workpiece with a pulsed laser beam intermittently emittedfrom a laser irradiation device, the improvement comprising: providing asolid transparent to the wavelength of the laser, serving as an entrancewindow to the surface of the liquid; allowing the liquid to be presentin the light path of the laser beam between the solid and the surface ofthe workpiece; and allowing the laser beam to enter through the solidand irradiating through the liquid the surface of the workpiece with thelaser beam, thereby shock-hardening the surface of the workpiece.

The present invention also provides in a laser shock hardening methodfor carrying out surface processing of a workpiece in contact with aliquid by irradiating through the liquid the surface of the workpiecewith a pulsed laser beam intermittently emitted from a laser irradiationdevice, the improvement comprising: filling the light path of the laserfrom the laser exit of the laser irradiation device to the surface ofthe workpiece with a liquid transparent to the wavelength of theirradiating laser; and irradiating through the liquid the surface of theworkpiece with the laser beam, thereby shock-hardening the surface ofthe workpiece.

The present invention also provides in a laser shock hardening methodfor carrying out surface processing of a workpiece in contact with aliquid by irradiating through the liquid the surface of the workpiecewith a pulsed laser beam intermittently emitted from a laser irradiationdevice, the improvement comprising: setting a velocity of relativemovement between the laser beam and the workpiece so that theirradiation interval of the laser beam applied to the surface of theworkpiece differs between the direction of the relative movement betweenthe workpiece and the laser beam and the direction perpendicular to therelative movement direction; irradiating through the liquid the surfaceof the workpiece with the laser beam emitted from the laser irradiationdevice; and moving the workpiece and the laser beam relative to eachother at the set relative movement velocity, thereby shock-hardening thesurface of the workpiece.

Preferably, the irradiation interval of the laser beam applied to thesurface of the workpiece is smaller in the direction of the relativemovement between the workpiece and the laser beam than in the directionperpendicular to the relative movement direction. Further, theirradiation interval of the laser beam in the direction perpendicular tothe direction of the relative movement between the workpiece and thelaser beam is preferably not more than 5 times the size of theirradiation spot of the laser beam.

The present invention also provides in a laser shock hardening methodfor carrying out surface processing of a workpiece in contact with aliquid by irradiating through the liquid the surface of the workpiecewith a pulsed laser beam intermittently emitted from a laser irradiationdevice, the improvement comprising: forming the laser beam emitted fromthe laser irradiation device in such a cross-sectional shape that alaser beam irradiation spot on the surface of the workpiece takes anelongate shape; irradiating through the liquid the surface of theworkpiece with the elongate irradiation spot; and moving the workpieceand the laser beam relative to each other, thereby shock-hardening thesurface of the workpiece.

Preferably, the laser beam irradiation spot has an elliptical orrectangular shape.

The present invention also provides in a laser shock hardening methodfor carrying out surface processing of a workpiece in contact with aliquid by irradiating through the liquid the surface of the workpiecewith a pulsed laser beam intermittently emitted from a laser irradiationdevice, the improvement comprising: positioning a tubular workpiececoaxially with the light axis of the laser beam; forming the laser beamemitted from the laser irradiation device in such a cross-sectionalshape that a laser beam irradiation spot on the inner peripheral surfaceof the workpiece takes the shape of a narrow ring; irradiating throughthe liquid the inner peripheral surface of the workpiece with the narrowring-shaped irradiation spot; and moving the workpiece and the laserbeam relative to each other in the axial direction of the workpiece,thereby shock-hardening the inner peripheral surface of the workpiece.

The present invention also provides a laser shock hardening apparatuscomprising: a laser irradiation device including a laser oscillator andan optical device for directing a laser beam, emitted from the laseroscillator, to the surface of a workpiece; a driving device for movingthe laser beam relative to the workpiece along the surface of theworkpiece; a solid member transparent to the wavelength of the laser,disposed at a position distant from the surface of the workpiece andserving as an entrance window to a liquid surface; and a vessel forfilling the light path of the laser beam between the solid member andthe surface of the workpiece with a liquid.

The present invention also provides a laser shock hardening apparatuscomprising: a laser irradiation device including a laser oscillator andan optical device for directing a laser beam, emitted from the laseroscillator, to the surface of a workpiece; a driving device for movingthe laser beam relative to the workpiece along the surface of theworkpiece; and a liquid jet nozzle, provided at the laser exit of thelaser irradiation device, for jetting a liquid transparent to thewavelength of the irradiating laser coaxially with the laser beam so asto fill the light path of the laser from the laser exit to the surfaceof the workpiece with the liquid.

The above laser shock hardening apparatuses may each further comprise ameans for controlling the velocity of the relative movement between thelaser beam and the workpiece so that the irradiation interval of thelaser beam applied to the surface of the workpiece differs between thedirection of the relative movement between the workpiece and the laserbeam and the direction perpendicular to the relative movement direction.

In a preferred embodiment of the present invention, the optical deviceincludes a means for forming the laser beam emitted from the laserirradiation device in such a cross-sectional shape that a laser beamirradiation spot on the surface of the workpiece takes an elongateshape. The optical device may include a cylindrical convex lens or acylindrical concave mirror, and form the laser beam, coming out of thecylindrical convex lens or the cylindrical concave mirror, in anelliptical cross-sectional shape and apply the laser beam to the surfaceof the workpiece. Alternatively, the optical device may include ahomogenizer for equalizing an intensity distribution in the laser beam,and a cylindrical convex lens or a cylindrical concave mirror, and formthe laser beam, coming out of the cylindrical convex lens or thecylindrical concave mirror, in a rectangular cross-sectional shape andapply the laser beam to the surface of the workpiece.

In a preferred embodiment of the present invention, the optical deviceincludes at the front end a rotationally-symmetrical mirror whichreflects the incident laser beam which is generally parallel to the axisof symmetry of the mirror to form a radial laser beam. Therotationally-symmetrical mirror may be a conical mirror and used incombination with a convex lens or a concave mirror.

Preferably, the line of intersection of the reflecting surface of therotationally-symmetrical mirror with a plane including the axis ofsymmetry of the mirror is part of a parabola, and the focus of theparabola lies approximately on the surface of the workpiece.

By providing a solid transparent to the wavelength of an irradiatinglaser at a distance from the surface of a workpiece and filling thelight path of the laser between the solid and the surface of theworkpiece with a liquid, according to the present invention, the liquidhas no free surface in the light path of the laser. In principle,therefore, there is no possibility of sputtering of the liquid andwaving of the liquid surface upon laser irradiation. It thus becomespossible to stably irradiate a predetermined point with a laser beam andto increase the processing speed by using a laser oscillator with ahigher repetition.

By filling the light path of an irradiating laser from the laser exit ofa laser irradiation device to the surface of a workpiece with a liquidtransparent to the wavelength of the laser, according to the presentinvention, the liquid has no free surface in the light path of thelaser. In principle, therefore, there is no possibility of sputtering ofthe liquid and waving of the liquid surface upon laser irradiation. Itthus becomes possible to stably irradiate a predetermined point with alaser beam and to increase the processing speed by using a laseroscillator with a higher repetition.

By providing a liquid jet nozzle at the laser exit of a laserirradiation device and jetting a liquid transparent to the wavelength ofan irradiating laser coaxially with a laser beam so as to fill the lightpath of the laser from the laser exit to the surface of a workpiece withthe liquid, according to the present invention, the liquid has no freesurface in the light path of the laser. Accordingly, it becomes possibleto stably irradiate a predetermined point with a laser beam and toincrease the processing speed by using a laser oscillator with a higherrepetition. Further, since the light path can be kept dean with theliquid jetted coaxially with the laser beam, there is no fear ofscattering or absorption of laser by impurities. Furthermore, aprocessing product produced upon laser beam irradiation can beeffectively removed by the jetted liquid. This makes it possible toapply laser pulses at a high repetition, thereby increasing theprocessing speed.

By making the irradiation interval of laser beam in the direction ofrelative movement between a workpiece and the laser beam smaller thanthat in the direction perpendicular to the relative movement direction,according to the present invention, it becomes possible to decrease thevelocity of the relative movement between the workpiece and the laserbeam, thus reducing the burden on the driving device. Accordingly, itbecomes possible to apply laser pulses at a higher repetition with thesame driving device used, thereby increasing the processing speed.

Furthermore, by forming a laser beam in such a cross-sectional shapethat a laser beam irradiation spot on the surface of a workpiece takesan elongate shape, and moving the workpiece and the laser beam relativeto each other in a direction generally perpendicular to the longdirection of the elongate irradiation spot, according to the presentinvention, it becomes possible to decrease the velocity of the relativemovement, thus reducing the burden on the driving device. Accordingly,it becomes possible to apply laser pulse at a higher repetition with thesame driving device used, thereby increasing the processing speed.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a conceptual diagram illustrating the principle of laser shockhardening;

FIG. 2 is a graphical diagram showing a change in the hardness of astainless steel by laser shock hardening;

FIG. 3 is a graphical diagram showing a change in the residual stress ofthe stainless steel by laser shock hardening;

FIG. 4 is a cross-sectional diagram illustrating a laser shock hardeningapparatus according to a first embodiment of the present invention;

FIG. 5 is a cross-sectional diagram illustrating a laser shock hardeningapparatus according to a second embodiment of the present invention;

FIG. 6 is a diagram illustrating a conventional manner of laser beamirradiation in laser shock hardening;

FIG. 7 is a diagram illustrating an irradiation interval betweenirradiation spots in laser shock hardening;

FIGS. 8A and 8B are diagrams illustrating different manners of laserirradiation according to a third embodiment of the present invention;

FIG. 9 is a diagram illustrating a yet another manner of laserirradiation according to the third embodiment of the present invention;

FIG. 10 is a graphical diagram showing the relationship between time andthe moving velocity of a driving device;

FIG. 11 is a graphical diagram illustrating a processing speed of lasershock hardening according to the third embodiment of the presentinvention;

FIG. 12 is a diagram illustrating the shape of an irradiation spot and amanner of laser irradiation according to a fourth embodiment of thepresent invention;

FIG. 13 is a cross-sectional diagram illustrating a laser shockhardening apparatus according to a fifth embodiment of the presentinvention;

FIG. 14 is a diagram illustrating the function of a cylindrical convexlens;

FIG. 15 is a cross-sectional diagram illustrating an irradiation headusing a cylindrical concave mirror, and its function;

FIGS. 16A and 16B are cross-sectional diagrams illustrating anirradiation head using a homogenizer, and its function;

FIG. 17 is a graphical diagram illustrating the effect of a homogenizeron equalization of laser intensity distribution; and

FIG. 18 is a cross-sectional diagram illustrating a laser shockhardening apparatus according to a sixth embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be describedwith reference to the drawings.

First Embodiment

FIG. 4 is an explanatory diagram illustrating a laser shock hardeningmethod according to a first embodiment of the present invention. Thesame members or elements as those of FIG. 1 are designated with the samereference numerals, and a duplicate description thereof will be omitted.

A workpiece 41 is set on a holder 42 placed in a liquid 22 filled in avessel 21. The holder 42, which fixes the workpiece 41, has a positionadjustment function of adjusting the height, angle, etc. of theworkpiece 42.

A pulsed laser beam 51, emitted from a laser oscillator 11, passesthrough a power adjustment device 12, a shutter 13 and an opticalinjection system 14 and enters an optical fiber 15. The laser beam 51emerging from the optical fiber 15 is directed by an irradiation head 17having a lens 16 toward the workpiece 41 in the liquid 22. While movingthe irradiation head 17 along the surface of the workpiece 41 by meansof a driving device 30, the laser beam 51 is intermittently applied tothe workpiece 41, thereby uniformly shock-hardening a predeterminedprocessing area of the surface of the workpiece 41.

A glass laser or a Nd: YAG laser, which oscillates at a near-infraredwavelength of about 1 μm, may be used for the laser oscillator 11. Whenwater is used as the liquid 22, the depth of the workpiece 41 in watershould be up to several mm because the near-infrared light is absorbableby water. However, in case the workpiece 41 has a complicated shape, itmay be difficult to control its in-water depth within several mm. It istherefore preferred to use for the laser oscillator 11 the secondharmonic wave of Nd: YAG laser (wavelength of 0.53 μm) which is littleabsorbed by water and thus is free of in-water depth limitation.

The power adjustment device 12 is an optical device which is comprisedof, for example, a combination of a polarizing plate and a branchingdevice, and which adjusts the energy of the laser beam 51. The shutter13 is comprised of, for example, a high-speed operational mirror and isso designed that through opening/closing control in synchronization withthe driving device 30, it allows the laser beam 51 to be applied only toa necessary portion of the surface of the workpiece 41. The opticalinjection system 14 adjusts and keeps the positional relationshipbetween the laser beam 51 and the optical fiber 15 and, with theprovision of a homogenizer or the like, also functions to flatten thein-beam spatial intensity distribution of the laser beam 51, therebypreventing damage to the laser inlet end of the optical fiber 15.

The irradiation head 17, provided with the lens 16, functions to directthe laser beam 51, emerging from the optical fiber 15, to the surface ofthe workpiece 41 while narrowing the laser beam 51. Accordingly, thearea of the irradiation spot on the surface of the workpiece 41 can bechanged and thus the peak power density (I(TW/m²)) of the laser beam 51applied to the surface of the workpiece 41 can be changed by changingthe distance between the irradiation head 17 and the workpiece 41.

The effect of laser shock hardening is determined by the pressure of theplasma 52 (P=(0.2×I)^(0.5)). In order to ensure the effect, it istherefore necessary to keep the peak power density within apredetermined range. For this purpose, the holder 42 for holding theworkpiece 41 has a position adjustment function, and makes a roughadjustment of the distance between the irradiation head 17 and theworkpiece 41 by adjusting the position of the workpiece 41. By alsocontrolling the height of the irradiation head 17 by means of thedriving device 30, the distance between the irradiation head 17 and theworkpiece 41 is kept within a predetermined range.

The height of the irradiation head 17 is controlled by measuring thedistance between the irradiation head 17 and the workpiece 41 with adistance measuring device (not shown), such as an ultrasonic distancemeter or a laser distance meter. The height of the irradiation head 17can also be controlled by the arrival time of generated sound of theplasma 52 generated by irradiation with the laser beam 51. It is alsopossible to control the height of the irradiation head 17 by measuringthrough the optical fiber 15 the reflection intensity of the laser beam51 reflected by the surface of the workpiece 41.

The plasma 52 generated by irradiation with the laser beam 51 loses itsenergy in a short time (about 10⁻⁷ second) and, as it is cooled, takesthe form of fine particles which float in the liquid. If the next laserbeam is applied to such a system, the fine particles will absorb orscatter the energy of the laser beam, precluding efficient laser shockhardening.

In this embodiment, in order to prevent floating of such fine particles,a liquid supply device 20 is connected to the vessel 21 so as tocontinually supply a fresh liquid 22 into the vessel 21. The liquidsupply device 20 is comprised of, for example, a pump, a filter and aflow meter.

Further in this embodiment, a transparent window 24, serving as anentrance window to the liquid surface, is set at approximately the samelevel as the liquid surface 23. A material transparent to the wavelengthof the laser used (i.e. little absorption of laser) suffices for thetransparent window 24. For example, quarts glass or polycarbonate,having excellent durability, may be used. By thus setting thetransparent window 24, the liquid 22 has no free surface in the lightpath of the laser beam 51. Accordingly, in principle, there is nopossibility of the occurrence of disturbance of the liquid surface 23.

If the transparent window 24 is not provided and the laser beam 51 isallowed to enter through the free surface of the liquid 22, sputteringor waving of the liquid 22 due to the pressure of the plasma 52 (thepeak pressure is about 3 GPa) will occur and, because of refraction ofthe laser beam 51, the position or the shape of the irradiation spotwill change. It is therefore necessary to await irradiation with thenext laser beam 51 until the disturbance of the liquid surface 23settles down. When, for example, using a vessel 21 of a size of 300mm×400 mm×200 mm (depth), it takes about 10 seconds for the disturbanceof the liquid surface 23 to settle down, precluding speeding up of theprocessing. The disturbance of the liquid surface 23 also entails theproblem that the sputtered liquid 22 adheres to the lens 16 of theirradiation head 17 or other optical devices, causing refraction of thelaser.

According to this embodiment, the provision of the transparent window 24prevents disturbance of the liquid surface 23 and thus can eliminate thewaiting time, making it possible to apply the puked laser beam 51intermittently at short intervals. Accordingly, it becomes possible toincrease the processing speed by using a laser oscillator 11 having ahigh repetition.

Though in this embodiment shown in FIG. 4 the transparent window 24 isprovided such that it covers part of the liquid surface 23, it is alsopossible to cover the entire liquid surface 23 with the transparentwindow 24. Further, though the optical fiber 15 is used as a means fortransmitting the laser beam 51, it is also possible to use anarticulated flexible arm used e.g. in the field of dental care.

In this embodiment a predetermined area of the surface of the workpiece41 is laser shock hardening-processed by moving the irradiation head 17.Needless to say, the same processing effect is achieved also in the caseof moving the workpiece 41 while the irradiation head 17 is fixed or inthe case of moving the irradiation 17 and the workpiece 41simultaneously.

Second Embodiment

FIG. 5 is an explanatory diagram illustrating a laser shock hardeningmethod and apparatus according to a second embodiment of the presentinvention. The same members or elements as those of the first embodimentare designated with the same reference numerals and a duplicatedescription thereof will be omitted.

The workpiece 41 in this embodiment is, unlike that of the firstembodiment, a structure which cannot be placed in the liquid in thevessel 21 in carrying out laser shock hardening, for example, a bridgepier.

In the case where the workpiece 41 is a bridge pier, the laser beam 51emitted from the irradiation head 17 is applied to a heat-affected zonearound a weld zone 43 to adjust the material characteristics includingresidual stress.

To obtain the desired effect of laser shock hardening, the irradiationportion 44 needs to be covered with the liquid 22 when it is irradiatedwith the laser beam 51. However, in the case where the irradiationportion 44 is positioned on the lower side of the structure, asillustrated in FIG. 5, it is difficult with the conventional techniqueto hold the liquid 22 in such a manner that it covers the irradiationportion 44.

Even in such a case, it becomes possible with the present invention tocover the light path of the laser beam 51 and the irradiation portion 44with the liquid 22 by jetting the liquid 22 from a liquid jet nozzle 61coaxially with the laser beam 51, as shown in FIG. 5.

After irradiation with the laser beam 51, the pressure of the plasma 52is transmitted through the liquid 22 to the liquid surface 23, causingsputtering of the liquid 22. For example, in the case where thethickness of the liquid 22 in the irradiation portion 44 is 1 mm, theliquid 22 begins to sputter about 10⁻⁶ second after irradiation with thelaser beam 51 whereby part of the irradiation portion 44 becomes exposedtemporarily. It is therefore necessary to re-cover the irradiationportion 44 with the liquid, e.g. by adjustment of the flow rate of theliquid 22, before irradiation with the next laser beam 51.

For the purpose of re-covering the irradiation portion 44 with theliquid 22 before irradiation with the next laser beam 51 to carry outthe laser shock hardening processing more continually, an experiment wasconducted with the size and the shape of the liquid jet nozzle 61 andthe jet flow rate of the liquid 22 as parameters to examine the timefrom when the irradiation portion 44 becomes exposed upon irradiationwith the laser beam till when the exposed irradiation portion 44 becomesre-covered with the liquid 22. The experiment was conducted by usingvarious repetitions up to 300 Hz to examine a change in the effect oflaser shock hardening with a change in the repetition.

It was confirmed from the results of the experiment that when the liquid22 is allowed to flow coaxially with the laser beam 51 e.g. at a speedof 3 m/s and a flow rate of 4 liters/min, the exposed irradiationportion 44 becomes re-covered with the liquid 22 about 10⁻³ second afterits exposure. It was also confirmed that an increase in the repetitionof laser oscillation up to a high repetition of 300 Hz produces nosignificant difference in the effect of laser shock hardening.

while it has been confirmed experimentally that the same laser shockhardening effect can be obtained with a repetition of laser oscillationup to 300 Hz by allowing the liquid 22 to flow coaxially with the laserbeam 51 at a speed of 3 m/s, as described above, it is calculativelypossible to increase the repetition of laser oscillation up to about 1kHz, thereby speeding up the processing. However, because of the factthat a pulsed-oscillation high-power laser currently availablecommercially has a repetition of 100 Hz at most, the 3 m/s flow speed ofthe liquid 22 suffices. Even when a laser oscillator that oscillates ata higher speed is developed by technological innovation in the future,the same laser shock hardening effect will be obtained by increasing theflow speed and the flow rate of the liquid 22.

In order to correctly apply the laser beam 51 to the irradiation portion44, it is important to avoid generation of a gas phase, such as airbubbles, in the liquid 22. For this purpose, it is necessary to controlthe flow speed and the flow rate of the liquid 22 to prevent the liquid22 jetted coaxially with the laser beam 51 from taking a negativepressure and causing cavitation before reaching the irradiation portion44.

Third Embodiment

FIGS. 6 through 11 are explanatory diagrams illustrating a laser shockhardening method according to a third embodiment of the presentinventions. The apparatus shown in FIG. 4 can be used for the lasershock hardening method of this embodiment. Thus, the same members orelements as those of the first embodiment are designated with the samereference numerals and a duplicate description thereof will be omitted.

The characteristic feature of the third embodiment of the presentinvention resides in the velocity of relative movement between a laserbeam and a workpiece. The moving velocity of the laser beam is relatedto how the laser spot moves on the surface of the workpiece. FIG. 6shows a distribution of irradiation spots 45 on the surface of theworkpiece 41 in laser shock hardening as carried out by the conventionaltechnique. FIG. 7 is a diagram illustrating an irradiation intervalbetween adjacent irradiation spots 45. While moving the irradiation head17 at a predetermined velocity in the lateral direction in FIGS. 6 and 7(X direction) by means of the driving device 30, the surface of theworkpiece 41 is sequentially irradiated with the laser beam 51 atregular irradiation intervals (dx). When the laser beam 51 has reachedthe boundary 47 of the processing area, the irradiation head 17 moves inthe vertical direction (Y direction) by a predetermined distance (dy),and again moves in the lateral direction (−X direction) while emittingthe laser beam 51 sequentially. This laser irradiation procedure isrepeated.

In the prior art, the irradiation interval (dx) of the laser beam 51 inthe moving direction of the laser beam 51 is made equal to theirradiation interval (dy) of the laser beam 51 in the directionperpendicular to the laser moving direction, so that the surface of theworkpiece 41 is irradiated uniformly and regularly with the laser beam51.

When the repetition of the laser oscillator 11 is increased in order tospeed up the laser shock hardening processing, it becomes necessary tomove the laser beam 51 at a higher velocity. In particular, the movingvelocity (v) of the laser beam can be represented by the followingequation, using the irradiation interval (dx) and the repetition (f) ofthe laser oscillator 11: v=dx f. The use of a high repetition (f) forspeeding up of the processing thus involves the need for an increasedmoving velocity (v) of the laser beam 51. This involves an increasedburden on the driving device 30, imposing a limitation on speeding up ofthe processing.

Another method conceivable for speeding up the processing is to increasethe pulse energy of the laser beam 51 to thereby increase the areaprocessible with one laser beam irradiation. This method, however,entails the problem that when the surface of the workpiece 41 is notflat, local variation (intensity difference) in the peak power densityof the laser beam 51 can be produced, making uniform processingdifficult. Furthermore, the use of a higher pulse energy requires theuse of a larger-sized optical transmission system including a mirror,making laser beam transmission by the optical filter 15 difficult.

According to this embodiment, in order to reduce the burden on thedriving device 30 which is an obstacle to speeding up of laser shockhardening, the irradiation interval (dx) of the laser beam 51 in thedirection of relative movement between the workpiece 41 and the laserbeam 51 is made smaller than the irradiation distance (dy) in thedirection perpendicular to the relative movement direction. Thus, thevelocity of relative movement between the workpiece 41 and the laserbeam 51 is made lower to reduce the burden on the driving device 30. Forsetting of such movement velocity and control of the driving device 30,the driving device 30 is provided with a control device 31.

FIG. 8A shows a distribution of irradiation spots 45 in the case ofdy/dx=4, and FIG. 8B shows a distribution of irradiation spots 45 in thecase of dy/dx=16. Though the number of pulses of the laser beam 51applied per unit area is the same as the prior art (FIG. 6), the movingvelocity of the laser beam 51 in the X direction becomes ½ (FIG. 8A) and¼ (FIG. 8B) of that of FIG. 6. Reduction of the burden on the drivingdevice 30 is thus evident.

In order to examine the effect of laser shock hardening according tothis embodiment in comparison with the prior art, an experiment wasconducted in which the surface of a stainless steel in water wasirradiated with a laser beam 51 having a pulse energy of 200 mJ and apulse width of 8 ns, with 36 pulses being applied per 1 mm², andresidual stresses in the surface and the interior of the stainless steelwere measured. The laser irradiation was carried out for five differentdy/dx rations 1, 4, 16, ¼ and 1/16, and three different diameters of thelaser spot 45, 0.6 mm, 0.9 mm and 1.2 mm, were tested for each dy/dxratio.

It was confirmed by the results of the experiment that the dy/dx ratiohas no influence on residual stress. In particular, with the same numberof pulses of the laser beam 51 applied per unit area, even a largedifference between dx and dy produces no difference in the residualstress distribution.

As illustrated in FIG. 9, as dx is further decreased, with the number ofpulses of the laser beam 51 applied per unit area constant, dy increasesin inverse proportion thereto and finally becomes larger than thediameter (D) of the irradiation spot 45 of the laser beam 51, resultingin the formation of gaps in the irradiation spots on the processingsurface. It has been confirmed experimentally, however, that even undersuch processing conditions, the residual stress becomes compressive atthe surface including gap portions when dy is 5 times D or smaller andthus a sufficient laser shock hardening effect is achieved.

In laser shock hardening, it is necessary to reverse the movingdirection of the driving device 30 at both ends of the processing area46, i.e. at the boundaries 47 of the processing area; andacceleration/deceleration upon the turn in direction is time-consuming.Therefore, the processing speed may not be greatly increased even whenthe oscillation frequency of the laser oscillator 11 is increased.

FIG. 10 shows the relationship between time and the moving velocity ofthe driving device 30 when the moving device 30 moves from one boundaryof the processing area 46 to the other boundary. The driving device 30,which started accelerating at T1, reaches a predetermined velocity (Vc)at T2 and keeps moving at the velocity, and starts decelerating at T5and stops at T6. At T3 immediately after the moving device 30 hasreached the predetermined velocity (Vc), the shutter 13 of the laserirradiation apparatus 10 is opened for irradiation with the laser beam51, and the shutter 13 is closed at T4. Next, the driving device 30moves in the Y direction by the distance dy, and then the driving device30 and the shutter 13 operate in the same manner as above. The aboveprocedure is repeated to process the processing area 46 sequentially.

FIG. 11 shows the results of determination of the relation betweenprocessing time 83 and dy/dx in laser shock hardening as carried out inthe above manner. The determination was made under the conditions of:the repetition of the laser oscillator, 300 Hz; the number of pulsesapplied per 1 mm², 36 pulses; the dimensions of the processing area 46,30 mm×30 mm; acceleration during acceleration/deceleration of thedriving device 30, 50 mm/S²; and the range of eachacceleration/deceleration area, 3 mm. The processing time 83 includeslaser beam irradiation time 81, acceleration/deceleration time 82 andY-direction movement time.

In the case of carrying out processing according to the prior art, i.e.with dy/dx=1, the processing time 83 is about 480 seconds about 80% ofwhich is spent on the movement of the apparatus. In the case of carryingout processing with dy/dx=4 according to the present invention, on theother hand, the processing time 83 is about 210 seconds which are almostequally divided into the laser beam irradiation time 81 and theacceleration/deceleration time 82. The processing 83 can be shortened toabout 140 seconds by carrying out processing with dy/dx=16.

As described hereinabove, by making the velocity of relative movementbetween the workpiece 41 and the laser beam 51 low according to thepresent invention, the burden on the driving device 30 can be reducedeven when the repetition of the laser oscillator 11 is increased tospeed up laser hardening processing.

Especially in carrying out laser shock hardening of e.g. an in-uselarge-sized structure by irradiating it with the laser beam 51 whilemoving the irradiation head 17, the moving velocity of the irradiationhead 17 can be made low according to the present invention. This makesit possible to use a smaller-sized lightweight driving device 30.Therefore, even in case of an accidental collision, for example due toan operational error, damage to the structure and to the driving device30 can be reduced.

In laser shock hardening of a workpiece in a narrow space, for example,a structure in a nuclear reactor, there is a case in which it isdifficult to process the entire processing area 46 with one type ofdriving device 30 or a case in which the entire processing area 46 isnot accessible from one direction, but only accessible by a plurality ofroutes. In such a case, processing is generally carried out in adivisional manner, with overlapped processing between divided processingareas being carried out. When processing a smaller processing area 46 asin this case, the acceleration/deceleration time 82 of the drivingdevice 30 relative to the laser beam irradiation time 81 becomes longer,leading to less efficient processing. The present invention isespecially useful for such processing.

Fourth Embodiment

FIG. 12 is an explanatory diagram illustrating a laser shock hardeningmethod according to a fourth embodiment of the present invention. Theapparatus shown in FIG. 4 can be used for the laser shock hardeningmethod of this embodiment. Thus, the same members or elements as thoseof the first embodiment are designated with the same reference numeralsand a duplicate description thereof will be omitted.

According to the above-described third embodiment, by making theirradiation interval (dx) of the laser beam 51 in the direction ofrelative movement between the workpiece 41 and the laser beam 51 smallerthan the irradiation interval (dy) in the direction perpendicularly tothe direction of the relative movement, the moving velocity of theirradiation head 17 and the driving device 30 can be decreased. Thisenables reduction of the burden on the driving device 30 and speeding upof laser shock hardening processing.

As described above with reference to FIG. 9, however, when the movingvelocity of the laser beam 51 is made extremely low, the interval (dy)between irradiation spots 45 in the Y direction becomes larger than thediameter (D) of the irradiation spot 45, resulting in the formation ofgaps, i.e. unprocessed portions, in the irradiation spots in theprocessing area 46. Though a sufficient laser shock hardening effect maybe achieved when the gap is 4 times the diameter (D) of the irradiationspot 45 or smaller, i.e. when dy is 5 times D or smaller, as describedabove, the effect can vary in places.

According to the fourth embodiment, the irradiation spot 45 of the laserbeam 51 on the surface of the workpiece 41 is formed in an elongateshape as shown in FIG. 12, and the laser beam 51 is moved in thedirection (X direction in FIG. 12) perpendicular to the long directionof the irradiation spot 45. This makes it possible to reduce the burdenon the driving device 30 and further increase the processing speed andto effect laser shock hardening without processing variation.

In accordance with the irradiation manner as illustrated in FIG. 12, alaser beam of a pulse energy of 200 mJ and a pulse width of 8 ns wascollected by a cylindrical convex lens so that the irradiation spottakes the shape of an ellipse having a major axis of 9.25 mm and a minoraxis of 0.2 mm and applied on the surface of a stainless steel at 36pulses per 1 mm². The residual stress at the surface after processingwas compressive, −645 MPa, and the residual stress had been changed fromtensile to compressive in the region to the depth of about 1 mm from thesurface. The residual stress improvement effect is similar to that shownin FIG. 3. The moving velocity of the laser beam 51 and thus of thedriving device 30 was 0.9 mm/s.

Table 1 shows comparison of the moving velocity of the laser beam 51according to the fourth embodiment with those of the prior art and thethird embodiment. According to the fourth embodiment, the movingvelocity of the laser beam 51 and thus of the driving device 30 is about1/56 of that of the prior art (dy/dx=1), and is 1/28 (dy/dx=4) or 1/14(dy/dx=16) of that of the third embodiment. Thus, the fourth embodimentof the present invention, while enjoying the same residual stressimprovement effect, can materially reduce the burden on the drivingdevice 30.

TABLE 1 Shape and size of Moving Processing Irradiation spot dy/dxvelocity Remarks conditions Circle 0.8 1 50 mm/s Prior art, Laserrepetition: FIG. 6 300 Hz 0.8 4 25 mm/s The present Number of pulsesinvention, applied: FIG. 8A 36 pulses/mm² 0.8 16 12.5 The presentinvention, FIG. 8B Ellipse 9.25 × 0.2  0.9 The present invention, FIG.12

When processing e.g. a heat-affected zone of a large-sized structureaccording to the fourth embodiment of the present invention, theintended processing can be completed by shaping the laser beam 51 intoan elliptical irradiation spot of a length of about 10 mm and moving thelaser beam along a weld line. In the prior art the movement of the laserbeam 51 is two-dimensional, and the stop (deceleration) operation andthe start (acceleration) operation of the driving device 30 are eachnecessary each time the laser beam 51 reaches the boundary 47 of theprocessing area. According to the present invention of this embodiment,on the other hand, such operations are necessary each only once in theabove processing, leading to speeding up of the processing and enhanceddurability of the driving device 30.

Furthermore, in the prior art the shape of irradiation spot 45 is acircle, for example having a diameter of 1 mm and, therefore, precisionof the order of ±0.1 mm is required in the movement of the drivingdevice 30 in the Y direction. On the other hand, when carrying outprocessing with an elliptical irradiation spot 45, for example having amajor axis of 10 mm, according to the present invention of thisembodiment, precision of the order of ±1 mm will suffice for themovement of the driving device 30 in the Y direction if such movement isnecessary. This materially increases design freedom for the drivingdevice 30.

Fifth Embodiment

FIG. 13 is an explanatory diagram illustrating a laser shock processingapparatus according to a fifth embodiment of the present invention. Theapparatus can be used for carrying out the above-described laser shockhardening method of the fourth embodiment. The same members or elementsas those of FIG. 1 are designated with the same reference numerals and aduplicate description thereof will be omitted.

The pulsed laser beam 51, emitted from the laser oscillator 11, passesthrough the power adjustment device 12, the shutter 13, a beam expander91 and a mirror 92 and enters the irradiation head 17 having acylindrical convex lens 93. The beam expander 91 functions to increaseor decrease the size of the laser beam 51 so as to adjust the size ofthe laser beam 51 entering the irradiation head 17.

FIG. 14 is an explanatory diagram conceptually illustrating the functionof the cylindrical convex lens 93 in this embodiment. The cylindricalconvex lens 93 is disposed such that its axis is generally orthogonal tothe laser beam 51. When the laser beam 51 entering the cylindricalconvex lens 93 has a circular vertical section, the cross section of thelaser beam 51 becomes elliptical on passing through the cylindricalconvex lens 93, and the irradiation spot 45 on the surface of theworkpiece 41 has the shape of an elongate ellipse.

In this embodiment the surface of the workpiece 41 is laser shockhardening-processed by moving a movable mirror 94 and the irradiationhead 17 at a predetermined velocity by means of the driving device 30.Upon the processing, the size of the irradiation spot 45 in the longdirection can be adjusted with the beam expander 91. If necessary, themovable mirror 94 and the irradiation head 17 may be movedtwo-dimensionally in a horizontal plane, or the workpiece 41 may bemoved by using the position adjustment function of the holder 42,whereby processing of a wider area can be carried out.

In this embodiment shown in FIGS. 13 and 14, the processing efficiencyis highest when the driving device 30 is moved in the directionperpendicular to the long direction of the elliptical irradiation spot45. Accordingly, when setting a desired moving direction of the drivingdevice 30, the cylindrical convex lens 93 may be rotated while keepingit coaxial with the laser beam 51 so as to carry out efficientprocessing.

FIG. 15 is a diagram illustrating the concept of the present inventionin the case of using a cylindrical concave mirror 95 instead of thecylindrical convex lens 93. The cylindrical concave mirror 95 has such ashape that the line of intersection with a plane parallel to the paperis part of a parabola and the line of intersection with a plane verticalto the paper is a straight line. Since an incident laser beam 51parallel to the axis of a parabola is reflected such that it is broughtto the focus, the use of the cylindrical concave mirror 95 can achievethe same effect as the use of the cylindrical convex lens 93 (FIG. 14).

FIGS. 16A and 16B are conceptual diagrams illustrating a manner ofprocessing the surface of the workpiece 41 more uniformly according tothis embodiment, both schematically showing the irradiation head 17provided with a homogenizer 97, as viewed from 90-degree differentdirections. As shown in FIG. 16A, the homogenizer 97 is an opticalcomponent in the shape of an obtuse-angled prism, and is so designedthat the right half and the left half of the incident laser beam 51 willoverlap each other in the irradiation spot 45.

FIG. 17 shows an intensity distribution of the laser beam 51 in theirradiation spot 45 as obtained by using the irradiation head 17 havingthe homogenizer 97, together with a comparative intensity distributionas obtained by using the irradiation head 17 without a homogenizer. InFIG. 17, the abscissa denotes position in the irradiation spot 45, andthe ordinate denotes the peak power density (relative density). Asapparent from the comparative data, the provision of the homogenizer 97can equalize the intensity distribution in the irradiation spot 45,providing a generally-flat intensity distribution.

Though in this embodiment the homogenizer 97 is disposed before (on thelaser oscillator side) the cylindrical convex lens 93, the same effectcan be obtained if the homogenizer 97 is disposed after the lens 93.Further, instead of the use as the homogenizer 97 of an opticalcomponent in the shape of an obtuse-angled prism, it is also possible touse a kaleidoscope, a microlens array, etc.

Sixth Embodiment

FIG. 18 is an explanatory diagram illustrating an irradiation head foruse in a laser shock hardening apparatus according to a sixth embodimentof the present invention. The irradiation head is used in place of theirradiation head 17 of the laser shock hardening apparatus of FIG. 1.The same members or elements as those of FIG. 1 are designated with thesame reference numerals and a duplicate description thereof will beomitted.

A workpiece 41 for use in this embodiment may be exemplified by theinner surface of a small-bore pipe. The irradiation head 17 is setgenerally coaxially with a tubular workpiece 41 by means of a not-shownpositioning jig. The laser beam 51 emitted from the laser oscillator 11is transmitted by the mirror 92, etc. and enters the lens 16 set insidethe irradiation head 17. The laser beam 51 which has passed through thelens 16 is gradually collected and reflected by a conical mirror 96 atan angle of approximately 90 degrees to become a radial laser beam 51,and the radial laser beam 51 is applied to the inner surface of thetubular workpiece 41. Reference numeral 98 denotes a cylindricalentrance window composed of a solid transparent to the wavelength of thelaser. Of course, the laser light path between the entrance window 98and the workpiece 41 is filled with a liquid.

When the focal length of the lens 16 is made almost equal to the opticaldistance between the lens 16 and the workpiece 41 by adjusting the focallength of the lens 16 or the setting position of the lens 16, theirradiation spot 45 on the surface of the workpiece 41 has the shape ofa narrow ring. The inner surface of the tubular workpiece 41 can beprocessed by irradiating the surface with such irradiation spot 45 ofthe laser beam 51 while moving the irradiation head 17 in the axialdirection of the tubular workpiece 41.

In the case of processing the inner surface of the tubular workpiece 41by the conventional technique, the laser beam 51 is emitted from theirradiation head 17 while rotating the irradiation head 17 at a highspeed and, at the same time, the irradiation head 17 is continuouslymoved in the axial direction of the tubular workpiece 17, so that theinner surface is irradiated with the laser beam 51 in a spiral manner.The conventional method thus necessitate the provision of a rotarysliding means, making the driving device 30 complicated. Furthermore,the conventional method involves a high operation speed of the drivingdevice 30 and thus a heavy burden on it, imposing a limitation onspeeding up of the processing.

According to this embodiment, on the other hand, the laser beam 51 isemitted radially whereby a simultaneous 360-degree processing can becarried out without rotating the irradiation head 17. There is,therefore, no need for high-speed rotational operation of theirradiation head 17 and thus no need for a rotational sliding means.This can materially simplify the construction of the driving device 30and can remarkably increase the processing speed.

Though in this embodiment the lens 16 is used in combination with theconical mirror 96 to form the radial laser beam 51, the same radiallaser beam can also be formed by using a concave mirror, instead of thelens 16, in combination with the conical mirror 96.

When the slope of the conical mirror 96 has such a shape that the lineof intersection of the slope with a plane including the axis of themirror 96 is part of a parabola, the incident laser beam 51 parallel tothe axis can be brought to the focus of the parabola and, therefore, thelens 16 becomes unnecessary.

1. In a laser shock hardening method for carrying out surface processingof a workpiece in contact with a liquid by irradiating through theliquid the surface of the workpiece with a pulsed laser beamintermittently emitted from a laser irradiation device, the improvementcomprising: setting a velocity of relative movement between the laserbeam and the workpiece so that the irradiation interval of the laserbeam applied to the surface of the workpiece differs between thedirection of the relative movement between the workpiece and the laserbeam and the direction perpendicular to the relative movement direction;irradiating through the liquid the surface of the workpiece with thelaser beam emitted from the laser irradiation device; and moving theworkpiece and the laser beam relative to each other at the set relativemovement velocity, thereby shock-hardening the surface of the workpiece.2. The laser shock hardening method according to claim 1, wherein theirradiation interval of the laser beam applied to the surface of theworkpiece is smaller in the direction of the relative movement betweenthe workpiece and the laser beam than in the direction perpendicular tothe relative movement direction.